Embodiments of the invention relate generally to power electronics systems and, more particularly, to control of power conversion in a power electronics system.
In power electronics systems, a power conversion process may include converting power from a source into a load power and supplying the load power to a load. In one example, a DC-link or voltage bus may supply the load power to a load coupled thereto such as connecting DC/DC converters to DC/AC inverters or other DC energy sources/sinks.
In an exemplary system, a hybrid electric vehicle may employ one or more common DC-link(s) coupled to available energy sources such as, for example, batteries, capacitors, flywheels, combustion engines, fuel cells, gas turbines, or the like. The DC-link voltage of the one or more common DC-link(s) should be kept within a defined operation band. This operation band could change depending on actual load on the traction inverter(s) due to efficiency optimization reasons. In other cases, it may be preferred to have the DC-link voltage at a constant value.
One method to hold the desired DC-link voltage is to exactly balance the power commands directed into the DC-link and coming out of the DC-link so that the energy in the DC-link remains constant. This strategy involves taking into account the dynamics of all involved converters as well as any communication delay/sample rate limitations that may be manifest in the supervisory controller. However, measurement errors (e.g., noise, offset), unknown dynamics (e.g., such as those due to nonlinearities in power conversion), and time delays of communication as well as sample and hold delay by the supervisory controller can limit an exact balancing of power into and out of the DC-link.
Typically, the DC-link is equipped with one or more capacitive devices (e.g., a battery or ultracapacitor) that provides substantial capacitance to filter the voltage ripple resulting from current ripple and to buffer energy in case of high frequency mismatch or imbalance of power flow into and out of the DC-link. The substantial capacitance helps buffer energy for preventing a drop/rise of DC-link voltage between subsequent or new power commands/demands of power conversion that may be due to an imbalance between power into and out of the DC-link. For example, a large capacitor may be used where voltage on the DC-link changes slowly compared to the power imbalance. Using a large capacitor, the supervisory controller is able to balance the voltage on the DC-link. However, these large-capacitance DC-link capacitors tend to be physically large components having a significant size and weight, which may be disadvantageous, especially for mobile applications such as in a hybrid electric vehicle application. In addition, these capacitors add extra costs to the systems/applications using them.
Therefore, it would therefore be desirable to provide an apparatus and system for controlling power conversion that reduces a capacitance, size, and weight of a capacitor used to buffer energy on a power conversion DC-link.